Many wastewater treatment plants use an activated sludge process to break down the organic matter in wastewater. A conventional activated sludge process typically begins with the removal of course materials such as grit, large particulate matter and other suspended solids from the wastewater. The wastewater is then sent to a primary clarifier where a sludge settles to the bottom and a clear supernatant sits on top. The sludge is removed for disposal or further treated in a digester, where a clear liquid is recycled back to the primary clarifier and unconsumed sludge is sent off for disposal. The clear supernatant from the primary clarifier is combined in an aeration tank with bacteria-laden sludge, also referred to as activated sludge, from a downstream process. The clear supernatant and activated sludge form a mixed liquor that is aerated in the aeration tank to reduce the BOD. The mixed liquor is then sent to a secondary clarifier to separate the sludge from a clear effluent. The clear effluent is removed, disinfected and typically discharged into a local waterway. A portion of the sludge is returned to the aeration tank as return activated sludge (RAS), while the remainder is usually concentrated and sent off for disposal as waste activated sludge (WAS).
Handling and disposal of WAS is typically the largest single cost component in the operation of a wastewater treatment plant. Therefore, the wastewater treatment industry would greatly benefit from a low WAS process where essentially all WAS was converted to a clear effluent that could be returned to a local water supply. One potential drawback to a low WAS treatment process, however, is the potentially high concentration of soluble phosphates left in the effluent. Phosphates are a common nutrient found in organic material making up WAS. As wastewater is subject to various treatment stages, the phosphates are either solubilized in the wastewater or taken up by microorganisms in the sludge. During a conventional activated sludge process, the phosphates are taken up by the microorganisms in the sludge and removed with the WAS. In the absence of WAS, the phosphates remain in the effluent as potential environmental pollutants. Therefore, any process that minimizes WAS should also include a process to eliminate the soluble phosphates remaining in the wastewater. Attempts have been made in the past to remove soluble phosphates from wastewater through precipitation with iron and aluminum metal salts. However, the resulting solid phosphates have settling properties similar to those of the organic solids and can be difficult to separate from the sludge in the wastewater. Additionally, such solids often become soluble at the lower pHs sometimes encountered in waste treatment processes.